In the present invention, processes are described which will reduce the energy required to smelt ores, and the processes will result in decreased expenditure for pollution abatement. Technology is available for smelting ores by heating them to high temperatures with a carbon source wherein the carbon combines with the oxygen associated with the ore to form carbon monoxide gas. However, large amounts of electric energy are consumed in smelting the ores. The technology can be advanced by developing processes and equipment to reduce the energy requirement. Energy can be conserved by effectively utilizing the potential energy in byproduct carbon monoxide gas.
The carbon monoxide gas forms in the high-temperature smelting zone of furnaces and it flows upward through the solid feed material. Heat is exchanged between the gas and the solids; the gas is cooled, and the solids are heated. This preheating of the feed mixture decreases the energy required for smelting. Present technology does not provide for efficient exchange of heat between the gas and feed mixture.
The invention is applicable to submerged arc electric furnaces operated as described above. These furnaces normally have three vertical electrodes that extend through the furnace roof into the feed mixture. Electric arcing occurs between the bottom of the electrodes and the furnace bottom. The furnace crucible is normally lined with carbon or graphite blocks joined with carbonaceous cement since this construction resists the high temperature and corrosive environment inside the furnace. FIG. 71 in the publication, "Production of Elemental Phosphorus by the Electric-Furnace Method," Chemical Engineering Report No. 3, Tennessee Valley Authority, National Fertilizer Development Center, Muscle Shoals, Ala. 35660, is a general arrangement of a submerged arc phosphorus furnace at which the electrodes are in line. FIG. 95 in the publication is a general arrangement of a round furnace at which the electrodes are disposed in a triangular arrangement. These diagrams show the geometry of the submerged arc electric furnace, and they may aid in understanding the present invention.
Ores smelted in submerged arc furnaces usually occur as small particles, but sometimes they occur as consolidated rocks which must be crushed and screened to obtain a size suitable for smelting. Fines are formed when the rocks are crushed and screened. The fines are unsuited for use in submerged arc furnaces because the gas velocity is high enough to suspend the small particles and they leave the furnace with the gas stream.
Technology is available to agglomerate discrete particles of ore. One process commonly used to agglomerate phosphate ore is called nodulizing. The ore is heated in a rotary kiln to a temperature high enough to melt part of the ore and thereby form a liquid phase. When unbeneficiated phosphate ore is nodulized the rotary kiln is heated to the temperature range of 2550.degree. to 2650.degree. F. Beneficiated phosphate ores must be heated to higher temperatures to obtain sufficient liquid phase for agglomeration, and the kiln lining is rapidly deteriorated. The mixture of melted and unmelted material is tumbled by rotation of the kiln forming agglomerates having a wide range of sizes. The proportion of liquid phase is poorly controlled and the kiln discharge will vary from unagglomerated fines to huge balls. Needless to say, large amounts of fuel are required to heat the kiln. The energy required for unbeneficiated phosphate ore is about 33 million Btu per ton of phosphorus produced. Of course, more energy is required for beneficiated ore.
Other agglomeration processes are available which require less energy, but more steps are involved in the process. The quality of the agglomerates may be no better than nodules. In one process moist ore is preformed into pellets, briquets, or other shapes. However, the agglomerates are too weak and they contain too much moisture to be fed to electric furnaces. The preformed shapes are indurated by heating to temperatures just below the point at which fusion begins. Fluorapatite mineral rapidly crystallizes and the agglomerates gain strength by crystal interlocking. The induration temperature for phosphate ore is in the range of 2200.degree. to 2300.degree. F. and the energy consumption is about 27 million Btu per ton of phosphorus produced. In addition, about 7.3 million Btu per ton of phosphorus is required for partial drying of unbeneficiated phosphate ore. The publication, "Agglomeration of Phosphate Fines for Furnace Use," Chemical Engineering Report No. 4, Tennessee Valley Authority, National Fertilizer Development Center, Muscle Shoals, Ala. 35660, provides information on the development of processes to agglomerate phosphate ores.
The term "high-temperature agglomeration" is used herein to denote those processes in which part of the ore is melted to form a liquid phase as in nodulizing. Or the term applies to processes wherein preformed agglomerates are indurated by heating to temperatures high enough to provide agglomerate strength by crystal interlocking. Fluorapatite begins to crystallize at about 1800.degree. F.
"Low-temperature agglomeration" is used to denote an agglomeration process disclosed in U.S. Pat. Nos. 4,372,929 and 4,373,893. Finely divided solids are tumbled in a rotating cylinder with a salt solution. The salt solution is formed in situ. The resulting agglomerates are indurated by drying at 250.degree. F., or higher. The salt is calcium phosphate or ammonium phosphate when phosphorus furnace feed materials are agglomerated. The energy to indurate agglomerates prepared from phosphate ore was estimated to be 7.3 million Btu per ton of phosphorus produced.
The equipment used in high-temperature agglomeration must be rugged in order to operate at elevated temperatures. The investment and operating costs are high, and air and water pollution abatement causes operating problems. The investment cost for high-temperature agglomeration at a phosphorus plant is about a third of the total cost. Small particles of ore are entrained in air during handling, resulting in a polluted workroom environment. Energy must be expended to treat large amounts of dust-laden air to remove dust for industrial hygiene control. Phosphate ores contain fluorine and part of the fluorine volatilizes when the ores are heated. Collection of fluorine from stack gases further contributes to the cost of air pollution abatement.
In high-temperature agglomeration the material must be cooled, screened, crushed, conveyed, and stored. Sometimes agglomerates are stored outside and this results in deterioration from exposure to the weather and from extra handling in taking material into and out of storage. Agglomerated ore contains a wide range of particle sizes and this results in the separation of large particles from small ones in storage, in the furnace feed bin, and in the furnace. This separation of the particles is called segregation.
Studies were undertaken to determine the magnitude of the segregation problem and to determine benefits that would result from feeding of uniformly sized materials. A special test was made at a 9,000-kW phosphorus furnace to obtain such data. The phosphate being smelted was agglomerated by nodulizing; the reducing carbon was metallurgical coke and the flux was silica rock. The nodulized phosphate had an average particle size of 0.8-inch, coke 0.3- to 0.4-inch, and the silica rock was slightly larger than the nodules. The furnace was being operated normally when it was shut down, and the mixture inside was sampled by taking core drill samples which were removed through three poke holes in the furnace roof. Results of the analyses of these samples are given in table 1.
TABLE 1 ______________________________________ Analysis of Samples of Core Drillings Taken from a 9,000-kW Phosphorus Furnace Distance from fur- P.sub.2 O.sub.5, Carbon, SiO.sub.2 :CaO nace roof percent percent weight ratio ______________________________________ Samples taken from poke hole nearest furnace offtake 4'5" to 5'1" 27.0 5.9 0.87 5'1" to 5'7" 27.4 4.2 0.78 5'7" to 6'0" 26.6 1.3 0.76 6'0" to 6'8" 26.0 0.7 0.82 6'8" to 7'5" 26.1 3.7 0.83 7'5" to 7'11" 23.0 11.5 0.90 7'11" to 8'2" 23.9 10.2 0.83 8'2" to 8'6" 23.4 12.4 0.84 8'6" to 8'10" 21.6 13.5 0.87 Samples taken from center poke hole 4'0" to 4'8" 26.4 7.6 0.70 4'8" to 5'3" 26.4 4.0 0.70 5'3" to 5'9" 26.7 2.1 0.77 5'9" to 6'3" 22.7 10.9 0.87 6'3" to 6'9" 22.0 15.1 0.88 6'9" to 7'2" 22.9 13.5 0.80 7'2" to 8'3" 16.2 21.0 0.94 8'3" to 9'8" 6.2 35.8 1.44 Samples taken from poke hole farthest from furnace offtake 6'0" to 6'5" 30.3 9.2 0.74 6'5" to 7'0" 27.7 5.7 0.77 7'0" to 7'5" 24.2 10.8 0.79 7'5" to 8'0" 22.0 16.8 0.76 8'0" to 8'8" 19.4 11.9 0.86 8'8" to 9'3" 17.0 13.4 0.86 ______________________________________
The chemical composition of the mixture inside the furnace varied widely as shown in table 1. The samples contained 19 to 304 percent of the carbon needed to react with the phosphate ore to produce elemental phosphorus. One sample had 1377 percent of the carbon needed, but it probably contained coke fines which had accumulated inside the furnace. Coke fines are ineffective for the reduction of phosphate. The fines collect in the bottom of the furnace and they are tapped out with the slag. The SiO.sub.2 :CaO weight ratio in core drill samples varied from 0.70 to 1.44, but the desired ratio was 0.85. The wide variations of both the carbon content and the SiO.sub.2 :CaO weight ratio were attributed to segregation of the materials wherein the phosphate, coke, and silica rock separated from each other because of their differences in particle size. The feed materials form conical-shaped piles when they are fed into the furnace through feed chutes; large particles roll down the surface of the cone, but small particles remain at the apex. It is impractical to install any mechanical devices inside the smelting furnace to prevent this coning.
Segregation was a subject of considerable study in the fertilizer industry because mixed fertilizers must be homogeneous to assure that the guaranteed nutrient contents of the mixtures are constant. It was found that segregation occurs when components of a solid mixture have different particle sizes. Under ideal conditions the average size and size distribution of the components of a solid mixture should be the same, and the mixture is said to have matched sizes.
A special test was carried out at a phosphorus furnace to investigate the benefits that might be realized by using matched sizes of materials as a feed mixture. Unbeneficiated phosphate ore was selected which had a SiO.sub.2 :CaO weight ratio of 0.85; therefore, no silica rock was needed. The ore was agglomerated by briquetting. The unbeneficiated ore contained enough clay to provide binder to make briquets which were indurated by heating them in a rotary kiln. Breakage of the briquets was severe, and it was apparent this method of agglomeration and induration would be impractical on a commercial scale. Nevertheless, sufficient material approximately 1 inch in size was obtained by screening to make a short furnace test. Large size metallurgical coke was purchased, and this material was crushed and screened so that particles about 1 inch in size were obtained. The phosphate ore had an average particle size of 1.02 inches and the coke had an average size of 1.01 inches.
FIG. 1 shows sections of the power chart prior to the test with matched sizes of materials and during this test. Prior to the test the phosphate was agglomerated by nodulizing and unmatched sizes of agglomerated ore, metallurgical coke, and silica rock were being fed to the furnace. The powerload varied widely as shown on the left-hand side of FIG. 1. There were deep dips to only 1,000 to 2,000 kW loads and numerous smaller dips to 5,000 to 6,000 kW loads. The average powerload was 13.8 percent less than the set load of 9,400 kW.
The right side of FIG. 1 shows the powerload with matched sizes of charge materials. The chart shows that no large dips occurred, and the average powerload was only 1.3 percent less than the set load. These data indicate that the capacity of a phosphorus furnace can be increased substantially by changing from unmatched to matched sizes of feed materials.
FIG. 2 is the pressure chart for the first day of the test and it shows that a marked difference was obtained when a change was made from unmatched to matched sizes of feed materials. Part of the chart shows pressure variations during normal operation when the ore was being agglomerated by nudulizing. The decrease in pressure variations when the change was made from nodules to briquets is readily discernible.
Furnace pressure fluctuations are an important index of of the performance. Wide fluctuations denote negative pressure part of the time, and air will be brought into the furnace through feed chutes and openings in the roof. Analysis of the furnace gas showed much less air was entering the furnace with matched sizes of feed materials than with unmatched sizes. Elemental phosphorus is oxidized to P.sub.2 O.sub.5 by the air and the P.sub.2 O.sub.5 is ultimately discharged as waste in condenser water. Electric energy was consumed in smelting the phosphate ore to reduce the oxidized phosphorus, and matched sizes of feed materials will therefore lower the electric energy requirement.
Positive pressure is accompanied by leakage of furnace gases through feed chutes and openings in the furnace roof. The escape of elemental phosphorus causes fumes in the workroom environment, as well as phosphorus loss. Most of the furnace gas is carbon monoxide and its escape into the workroom can be dangerous. However, use of matched sizes of feed materials substantially reduces the pressure surges which can cause a dangerous environmental condition.
Wide variations in pressures indicate that the gas is channeling through the feed materials inside the furnace. The exchange of heat between the hot gas and the feed mixture is poor. With matched sizes of feed materials the furnace gas temperature held steady at about 475.degree. F., which was about 200.degree. F. lower than the average temperature with unmatched materials. Therefore, additional electric energy is saved by improved heat exchange inside the furnace.
Phosphorus furnace gases contain elemental phosphorus and noncondensable gases, but most of the noncondensable gas is carbon monoxide. About 6 percent of the gas by volume is phosphorus and the dewpoint is 460.degree. F. The furnace gas temperature must be kept above this value to avoid problems from the condensation of phosphorus inside the furnace, in the furnace offtake, and in the electrostatic precipitator. A minimum furnace gas temperature of 500.degree. F. is proposed to provide a margin of safety of about 40.degree. F.
Heretofore, phosphorus furnaces were constructed with depths which would assure that the dewpoint of phosphorus was not reached, but the average gas temperature was substantially above the dewpoint. With the matched sizes of feed materials it will be possible to design for steady gas temperatures and the difference between the average gas temperature and the dewpoint can be decreased. Additional heat can be extracted from the furnace gas and less electrical energy will be required for smelting.
Various carbonaceous materials are capable of reacting with phosphate ores to produce elemental phosphorus. But many of the materials are unsuited as reducing carbons because they are comprised of small particles. Metallurgical coke is commonly purchased for use as a reducing carbon in phosphorus furnaces. Unfortunately, metallurgical coke usually contains about 12 percent of material smaller than 10 mesh, and use of the fine coke adversely affects the furnace operation. Furthermore, the small sized coke is ineffective as a reducing carbon.
Practical processes were not available for the agglomeration of small sized carbonaceous materials prior to disclosures in U.S. Pat. No. 4,421,521, Dec. 20, 1983. Metallurgical coke fines can be agglomerated by the low-temperature process described above to make particles large enough to be used as a reducing carbon. When the reducing carbon is metallurgical coke, about 80.9 million Btu of energy are required in smelting to produce a ton of elemental phosphorus. Forty-seven percent of the energy is metallurgical coke and 53 percent is electric energy. An energy saving of about 4.6 million Btu per ton of phosphorus can be obtained by agglomeration of the coke fines. Additional energy savings may be obtained by agglomerating low-cost carbonaceous materials and using these materials as reducing carbons. For example, agglomerated anthracite coal fines can be used as a reducing carbon and cost of phosphorus production will be further reduced.